1. Field of the Invention
The present invention concerns an optical material of excellent transparency, heat resistance and impact resistance, having low specific gravity, and reduced weight. The invention provides a molding product of high refractive index, an optical molding product using such an optical material and a method of manufacturing thereof. The optical molding product according to the present invention is useful as various kinds of lenses used for optical instruments or spectacles and, in addition, as prisms, optical waveguides, disc substrate or the like.
2. Description of the Prior Art
Materials used to produce optical members such as lenses, prisms, optical waveguides and disc substrates need to be colorless and transparent. Furthermore, by reason of their reduced weight, (satisfactory) impact resistance, moldability or dyeability, transparent synthetic resins have attracted attention as a substitute for inorganic optical material.
Various characteristics are required for transparent synthetic resins as optical materials in addition to the above, and the refractive index is extremely important among them. For example, transparent synthetic resins having a high refractive index, when used as lenses, can be rendered thinner than materials having low refractive indexes to give the same focal distance. Accordingly, it can reduce the volume of space occupied by lenses in optical assemblies thereby reducing the weight and minimizing the size of optical apparatuses. Furthermore, since transparent synthetic resins have excellent impact resistance as compared with inorganic optical materials such as glass, they can be considered also excellent in durability.
Diethylene glycol bisallyl carbonate resin, polymethylmethacrylate and polycarbonate have been generally known as resins to be used for the optical material such as plastic lenses. However, since the diethylene glycol bisallyl carbonate resin and the polymethylmethacrylate have low refractive indices of 1.49 to 1.50, when these resins are molded into plastic lenses, they bring about a drawback that a center thickness, an edge thickness and a curvature of the lens become greater as compared with those of inorganic optical glass lenses. Further, although the polycarbonate resin has a high refractive index of 1.58 to 1.59, it is liable to cause birefringency in molding and thus is defective in optical homogeneity. Moreover, because polymethylmethacrylate and polycarbonate are thermoplastic resins of non-crosslinked structures, the resins are fused during cutting or grinding and they can not be considered satisfactory as materials for ranges in precision optical machinery, optical elements or ophthalmic lenses.
To remedy the above drawbacks of the thermoplastic resins, a method has been so far known which produces resins having a crosslinked structure using ethylene glycol dimethacrylate as a crosslinking agent (Japanese Laid-Open Patent Application No. 64691/1974), but the resin of such a cross-linked structure has low impact resistance.
Further, there is also a method of manufacturing a resin of a crosslinked structure by using trimethylol propane tri(meth)acrylate (Japanese Laid-Open Patent Application No. 291841/1988) but the resin material can not be put to practical use as the optical material since it has a poor transparency being prepared by curing with dispersed metal oxide hydrates.
Further, Japanese Laid-Open Patent Application No. 34102/1987 discloses a styrene derivative having a halogen-substituted aromatic ring as a component having a high refractive index. However, use of the halogen-substituted styrene derivatives involves a problem that the specific gravity is increased and the weather proofness is poor as the optical material.
In order to overcome the foregoing drawbacks, optical material and optical molding products using a resin of high refractive index have been developed, for example, for ophthalmic plastic lenses. For producing an optical molding product such as plastic lenses by using a resin of high refractive index, it has been adopted a cast polymerization process of conducting polymerization while casting, into a molding die, a polymerizable ingredient having halogen atoms such as chlorine or bromine atoms; nitrogen atom-containing ingredient such as urethane; sulfur atom-containing ingredient such as thiol; or an aromatic ring or the like in the molecule, for example, vinyl monomer, prepolymer or (poly) condensation type monomer.
However, the method of producing an optical molding product having a high refractive index by the cast polymerization of the polymerizable ingredient described above shows a poor molding yield as compared with the case of using the allyl carbonate or the like as the material. Generally, the polymerizable ingredients as described above involve a difficulty that the control of polymerization is difficult compared with allyl carbonate, strains are left in the optical molding product and thus the adhesion between it and the mold becomes insufficient and the ingredients separate off the molding die.